1. Field of the Invention
This invention relates to optical devices and in particular to devices relying on electrochromic effects.
2. Art Background
Electrochromic devices such as those based on WO.sub.3 and iridium oxide have been contemplated for use in electro-optic applications. Such electrochromic cells have many outstanding features, e.g., passive long-term memory, the possibility of either black or colored states, wide viewing angle, and sharp electrical thresholds for switching, that make their development of significant interest. However, a number of electrochromic materials with desirable chemical and electrical properties are, nonetheless, too weakly absorbing in the visible wavelength region to produce electro-optic devices with high or even adequate optical contrast.
The contrast of a non-emissive optical device is the difference in intensities of externally incident light reflected or transmitted by the device in its bleached and colored states. This contrast is typically quantified in terms of a contrast ratio--the ratio of the light reflected or transmitted by the device in its bleached state to that reflected or transmitted in its colored state. Electrochromic devices having desirable electrical and chemical properties typically have contrast ratios in the range between approximately 2 to 1 and 4 to 1. Although this level of contrast is adequate for some applications, for devices with more demanding requirements, e.g., multielement information displays, higher contrast ratios are desirable, to allow ready visual resolution and identification of characters and to avoid reader fatigue.
Researchers have attempted to ahieve increased contrast by searching for new electrochromic materials that inherently yield the desired improvement. This approach, however, has not been particularly successful. The search for new electrochromic materials having the desired chemical and electrical properties and that yield large optical absorption changes has been time consuming and generally unavailing. Therefore, electrochromic devices are not preferred in certain applications where high contrast is necessary.